This invention relates to methods for controlling the operation of a fluid machine in spinning reserve for practicing a phase modifier operation or for producing a so-called standby condition which permits immediate switch-over to a power-generating or pumping-up operation, and more particularly to a method of the type described which has the effect of avoiding accidents due to a rise in the temperature of the fluid machine.
In a fluid machine, such as water turbine or reversible pump water turbine, which is connected directly to a synchronous generator, a so-called phase modifier operation is practiced for improving the power factor of the transmission system when such factor is lowered. In practicing the phase modifier operation, the water level in a runner chamber is forced down and the guide vane and inlet valve are fully closed to rotate the runner in compressed air atmosphere by introducing compressed air into the runner chamber. When a fluid machine practices such pattern of operation, the fluid machine is said to be in spinning reserve condition. A fluid machine is also brought to the condition of spinning reserve for producing a standby condition which permits immediate switch-over of the fluid machine to a power-generating operation. A fluid machine may remain in the spinning reserve condition for several hours to several scores of hours.
A method of the prior art for operating a fluid machine in the spinning reserve condition will be described in detail. The guide vane and inlet valve are fully closed and the water level in the runner chamber is forced down by introducing compressed air thereinto, so as to rotate the runner in compressed air atmosphere. Inasmuch as the inlet valve is fully closed, water leakage from the guide vane causes a reduction in the hydraulic pressure in the casing, thereby permitting the air in the runner chamber to be introduced into the casing. In order to prevent a reduction in the hydraulic pressure in the casing, a valve mounted in a line connecting the casing to a penstock on the upstream side of the inlet valve for supplying water to compensate for the leakage of water from the guide vane is opened to supply water to the casing, so as to maintain the pressure within the casing at a level which is slightly higher than the pressure within the runner chamber. It is for the purpose of preventing damage to an intake and a screen or the like arranged at the intake which would be caused by the explosion of air flowing upwardly through the penstock from the casing when a pumping-up operation is initiated or the fluid machine is switched to a power-generating operation from a standby condition, that the introduction of air into the casing is prevented.
Since the pressure within the casing is higher than the pressure within the runner chamber, water leaks slightly from the guide vane. Also, a seal provided between the outer circumferential surface of the runner and fixed parts has a gap which is small in size, so that cooling water is supplied from a device specially provided for that purpose to the outer circumferential surface of the runner to prevent heating thereof. As a result, a wall of water is formed around the outer circumferential surface of the runner due to collection of such water. If the runner is rotated in compressed air atmosphere under such condition, blades of the runner will vigorously stir up the water therearound. This will increase an input to the shaft required for rotating the runner in compressed air atmosphere and impart energy of a high value to the water. Part of the energy will raise the temperature of the water around the runner, part thereof will be dissipated to the surroundings by heat transfer, and the remainder will be released to outside by the leakage water discharge through a leakage water discharge pipe. When the runner rotates in compressed air atmosphere in the spinning reserve condition, the temperature of the water around the runner rises as aforesaid. No serious problems are raised so long as the period of time for spinning reserve of the fluid machine is short. However, prolonged holding of the fluid machine at spinning reserve condition causes a great rise in the temperature of the water. In particular, even if a method is used whereby the difference between the pressure within the casing and the pressure within the runner chamber is minimized for reducing the amount of water leaking from the guide vane when a fluid machine is placed in spinning reserve condition, there are limits to the quantity of water discharged through the leakage water discharge pipe, and it is impossible to raise the quantity of discharged leakage water to a very high level. Thus, prolonged holding of the fluid machine at spinning reserve condition results in gradual collection of water around the runner after such water leaks from the inlet valve or the guide vane. This means an increase in the power required for driving the runner to rotate the same and an increase in the value of energy imparted to the water around the runner, thereby greatly raising the temperature of the water.
If the water around the runner has a very high temperature, the runner will undergo a high degree of thermal expansion and will be brought into contact with the fixed parts during its rotation. Thus an accident will occur involving the scraping of the surrounding parts by the runner which will cause damage to the rotor. Moreover, since the power required for rotating the runner is high, there will be a loss of energy and the efficiency of operation of the power plant will be reduced. If it is desired to reduce to zero the quantity of water leaking from the guide vane, the end can be attained by fully closing the inlet valve and discharging water from the casing by using compressed air to fill the casing with compressed air in the same manner as the runner chamber is filled with compressed air. However, this proposal will be impracticable, because the casing has a volume which is about three times as large as that of the runner chamber and the quantity of compressed air required for discharging water from the casing will be immense. This means that water level forcing down equipment and a compressed air producing device will have to be of a very great size, causing an increase in the size of a power generating plant. In an underground type generating plant used in a pumping-up power plant system, the earth excavated will increase in volume and cause a fatal blow to this type of power plant system from the economical point of view.